Method of introducing active metal into electric discharge devices



June 26, 1934.

J. H. DE BOER ET AL 1,964,506

METHOD OF INTRODUCING ACTIVE METAL INTO ELECTRIC DISCHARGE DEVICES Filed Nov. 18, 1931 INVENTOR5 JAN HENDRIK DE BOER JOHAN LODEWYK HENDRIK JONKER BY KARL yp g QNSESSEL TTdRNEY M Patented June 26, 1934 METHOD OF INTRODUCING ACTIVE DIETAL INTO ELECTRIC DISCHARGE DEVICES ware Application November 18, 1931, Serial No. 575,848 In the Netherlands November 18, 1930 2 Claims.

This invention relates to a process of introducing a chemically active metal into a vacuum tube. Such chemically active metals among which the alkaline earth and alkali metals play an important part are inserted into vacuum tubes for difierent purposes, for example, for forming an electron active coating on an incandescent cathode or for taking up residual gases or gaseous impurities.

On account of their other great aflinity for oxygen these metals cannot be exposed to open air and for this reason various methods have been adapted for introducing the metals into a vacuum tube without running the risk of their combining with oxygen from the air.

One method consists in introducing into the tube a compound of the active metal which upon heating will free the metal and which is not itself subject to be attacked by oxygen. In this method use is made essentially of active metal azides which decompose upon heating and thus set free the active metal with the development of nitrogen. Very satisfactoryresults have been obtained by this method.

In order to avoid the necessity of removing the nitrogen developed it has been suggested that an alkaline earth oxide, for example, barium oxide, be admixed with aluminium and that this mixture be so treated as by heating it outside the discharge tube in which the barium is to be inserted, that the barium oxide is reduced by the aluminium. The mixture is heated to such 7 a temperature that the reduction ensues with the formation of barium metal and aluminium oxide but the barium formed does not volatilize or v0l atilizes only partially. In this case the remaining reaction-products are left in the form of a mixture of aluminium oxide and barium. This mixture is introduced into the discharge tube and after the exhaustion of the tube the mixture is heated to such a temperature that the barium is expelled.

Since aluminum readily alloys with alkaline earth metals a considerable part of the barium developed during the treatment outside the dis charge tube will alloy with the aluminum. The aluminum-barium alloy thus formed and then introduced into the discharge tube is decomposed only at a higher temperature than that at which .barium is liberated from a mixture of barium oxide and aluminum oxide so that an important advantage of the method described is lost. By reason of the higher heating required there is also the risk that the heating may be carried so high that although aluminum has a higher temperature of volatilization than barium part of the aluminum volatilizes and deposits also on those parts of the discharge tube where barium is to be precipitated. This may be very detri mental, for example, when the barium is deposited on a cathode, for if in this case the barium is admixed with a little aluminum, the presence. of the latter metal may cause a considerable reduction of the electron emission of the cathode.

According to the invention these diificulties are obviated by introducing the active metal into the vacuum tube after admixture with an insulating compound of an alkaline earth metal, such' as calcium compound. This mixture can be prepared by admixing a compound of the active metal to be inserted with calcium and heating this mixture in a vacuum or in a neutral atmosphere to such a temperature that the compound of the active metal is reduced and a mixture of the active metal to be inserted and a calcium compound is formed, as calcium does not alloy to any serious extent with this active metal or at least forms alloys which readily decompose so that the mixture after its introduction into the vacuum tube need be heated to but a comparatively low temperature for causing the active metal to volatilize. Moreover it is possible by gently heating this mixture to expel the active metal slowly from the mixture, other substances in the mixture being thus prevented from volatilizing simultaneously. If nevertheless a little calcium does volatilize and deposit, for example, on a cathode it has practically no injurious effect as the electron emission of the cathode is not lowered. Preferably, the active metal to be inserted will be admixed with calcium oxide and for this purpose a mixture of barium oxide and calcium will be initially used and by heat converted at least partially into barium and calcium oxide.

It is often desirable that the mixture to be introduced into the tube should contain such a quantity of calcium oxide and sometimes also of another insulating substance refractory enough to stand the heating so that the mixture is electrically insulating. Thus the advantage is ob-- tained that if particles of the mixture shaped, for example, into the form of a pellet should become loose, these particles cannot establish conductive connections between live parts. The insulating substance may consist only of calcium oxide but the mixture may contain in addition other insulating substances preferably refractory oxides such as magnesium oxide or zirconium oxide if initially for the preparation of the mixture a plentiful supply of barium oxide is used so that this can be reduced only partially by the calcium, the mixture introduced into the tube will also contain barium oxide which is substantially an insulator when cold.

The mixture of the calcium compound and the active metal to be introduced into the vacuum tube may have added to it a binder and maybe kept in a vacuum or under an air-excluding liquid. It is possible to expose the mixture for a short time to open air without the active metal being attacked to any serious extent by the oxygen of the air.

In order that the invention may be clearly understood and readily carried into eifect one embodiment thereof will now be described more fully by way of example with reference to the accompanying drawing, in which Figs. 1 and 2 show an electric discharge tube the cathode of which is superficially coated with barium.

The discharge tube shown in the drawing has a glass envelope 1 and is adapted to receive or to amplify electric oscillations. The tube contains an anode 2, a grid 3 and an electron emit-.

ting cathode 4. The tube is constructed in the usual manner, hence its construction need not be set out more fully. The anode has an aperture 5 in front of which is arranged externally to the anode a capsule 6 fixed to the anode and if necessary closed by a gauze. of the electrodes in the envelope this capsule is filled with a mixture of barium and calcium oxide and after the exhaustion of the envelope by a pump the capsule is heated by suitable means for example, by a high frequency magnetic field to' a temperature at which barium is set free in the form of vapour. The barium is deposited on the cathode and forms there, sometimes after oxidation, a layer of high electron emitting power. The barium vapour is expelled at a comparatively low temperature. 'The calcium oxide prevents an excessively rapid formation of vapour and has itself such a low vapour pressure that it does not produce any harmful vapours in the tube.

For the purpose of preparing the mixture to be introduced into the tube, a small quantity of barium oxide and metallic calcium may be Prior to the mounting brought together in a glass vessel which is subsequently exhausted by'a pump. After that the communication between the vessel and the vacuum pump is broken and the mixture present in the vessel is brought to a temperature of about 806 C. and the barium oxide is reduced by the calcium, at least partially with the formation of metallic barium. The temperature to which the mixture is heated is kept so low that a large portion of the barium developed during the reduction remains so that a mixture is formed which contains barium and calcium oxide. This mixture can be kept in the vessel used in preparing it or in a receptacle under an air-excluding liquid. When the mixture is being introduced into the discharge tube it can be exposed to air for some time, as the calcium oxide forms a satisfactory cover so that at the most only a little barium at the surface will be attacked by air. 'If it is desired that barium oxide should not be initially used, use may be made of other salts of barium for example of barium fluoride, and this compound may be reduced by means of calcium. In this case the mixture to be introduced into the tube contains barium and calcium fluoride.

It is not necessary that the mixture containing the active metal should be introduced into the discharge tube itself as it may be introduced into a separate chamber communicating with the tube. In this case, the vapour of the active metal developed in this chamber can flow from this chamber into the discharge tube.

I claim: I l. A clean up agent for use in electric discharge devices consisting'of an electrically insulating mixture of barium, calcium oxide, and barium oxide.

2. An electron discharge tube comprising a plurality of electrodes including a plate, a capsule carried by the plate, and a mixture, introduced into said capsule in the form of a pellet, of barium, calcium oxide and a barium compound in such proportions that said mixture is substantially electrically insulating.

JAN 'HENDRIK DE BOER. J OHAN LODEVVYK HENDRIK JONKER. KAREL lVIARINUS VAN GESSEL. 

